1. Field of Invention
The present invention relates to a color television system. More particularly, the present invention relates to an apparatus and method for producing sub-carrier signals.
2. Description of Related Art
Receivers for different television systems, such as the digital color signal reproducing circuits often used in NTSC and PAL television systems, have become widely used with the popularity of televisions and offered increasingly better performance, as technology progresses. FIG. 1 is a schematic view of a conventional digital phase-locked loop (PLL) circuit, which is applicable to NTSC/PAL/SECAM television systems. The function of the digital phase-locked loop circuit 100 is to generate sub-carrier signals for a subsequent color demodulator according to the inputted color burst signals.
As illustrated in FIG. 1, a phase detector (PD) 102 compares phases of the color burst signal and the sub-carrier signal to generate a phase shift signal. A loop filter 104 filters the phase shift signal, and then the filtered phase shift signal is inputted into a digital control oscillator (DCO) 106.
The digital control oscillator 106 generates the sub-carrier signals according to the filtered phase shift signal and feeds back the generated sub-carrier signals to the phase detector 102. With the feedback procedure, the digital phase-locked loop circuit 100 can generate the sub-carrier signals of the same oscillation frequency with that of the color burst signals and lock the phases of the color burst signals and the sub-carrier signals such that there is no phase shift between the two signals.
However, in the conventional digital phase-locked loop circuit 100, the phase detector 102 may lock the phases of the color burst signal and the sub-carrier signal erroneously at 180 degrees instead of at the correct 0 degree when the phase shift between the two signals is over 90 degrees. FIG. 2 is a schematic diagram of the opposite phase locking of the color burst signal and the sub-carrier signal. As illustrated in FIG. 2, the foregoing condition causes the color burst signal 202 (solid line) and the sub-carrier signal 204 (dashed line) to be locked in the undesirably opposite phases when they are of steady states, and thus the subsequent color demodulator cannot obtain correct colors by reference.
The prior art provides several different methods or apparatuses to solve the problem of erroneous phase locking. For example, Taiwan Patent No. 373,389, “A video input apparatus for connecting to a computer and the method thereof”, disclosed an apparatus and method using table lookup to solve the erroneous phase locking. U.S. Pat. No. 6,741,289, “Technique to stabilize the chrominance subcarrier generation in a line-locked digital video system”, and No. 6,310,653, “Phase comparison and phase adjustment for synchronization to a reference signal that is asynchronous with respect to a digital sampling clock”, both solve the problem with very complicated circuits.
Other conventional techniques, such as U.S. Pat. No. 6,538,702, “Digital color signal reproducing circuit”, and No. 6,034,735, “Clock generator for digital video signal processing apparatus”, solve the problem by phase compensating, but they still require large circuits and cannot easily complete the phase locking within a short time.